1. Field of the Invention
The present invention relates to a video signal processing apparatus and method, in particular, a video signal processing apparatus and method for converting plural input video signals to other video signals to output the converted signals.
2. Description of Related Art
In the past 50 years, an analog television broadcasting service has been provided in the USA via land broadcasts, cable, or other such media based on an NTSC (National Television Standards Committee) system.
A video signal based on the NTSC system is described. FIG. 7 shows a waveform of the video signal based on the NTSC system. This video signal includes a video signal component where a luminance signal (Y signal) and a chrominance signal (C signal) are multiplexed, a burst signal used as a reference for the chrominance signal, and a horizontal synchronizing signal for horizontal scanning, and is called a composite signal.
Further, this video signal has a frequency characteristic as shown in FIG. 8. The video signal is within a frequency band from 0 to fmax (maximum frequency), and a luminance signal falls within such a frequency band. The chrominance signal is modulated with a color subcarrier SC of a color subcarrier frequency fsc, and its frequency is within a frequency band spread from the color subcarrier frequency fsc by a predetermined value. This color subcarrier frequency fsc is also a frequency of the burst signal of FIG. 7. Considering the NTSC video signal, for example, fmax≅4.2 M Hz, and color subcarrier frequency fsc=3.579545 MHz.
A television receiver conforming to the NTSC system receives such a video signal, separates the video signal into a luminance signal and a chrominance signal (Y/C separation), and demodulates the chrominance signal (chroma demodulation) to display moving pictures. In general, the Y/C separating ability of the television receiver has a limit, so the separated Y signal components and C signal components involve crosstalk. In particular, the Y signal represents the luminance level, while the C signal is a signal demodulated with the color subcarrier SC (burst signal), the C signal causing crosstalk with the Y signal appears to be consecutive dot-like interfering wave (dot interference) in the human eyes. To that end, in the NTSC system, the following devises are made for the purpose of reducing the interference due to the crosstalk.
FIGS. 9A and 9B show a relation between a scanning line and a color subcarrier in an NTSC video signal. In the NTSC system, interlacing scanning is adopted to scan a frame as a single image which is divided into two fields. For example, first scanning displays even-numbered lines, and second scanning displays odd-numbered lines. One frame has 525 scanning lines, so one field has 262.5 scanning lines.
FIG. 9A is a field perspective view and sectional view showing a phase shift of a color subcarrier in each field based on the NTSC system, and the fields are arranged in order of display time. In FIG. 9A, 5 fields (M+0-4), that is, 2.5 frames are arranged in turn. In FIG. 9A, to focus the description on the phase shift of the color subcarrier SC, only the color subcarrier SC repeatedly appears. In practice, however, the waveform of FIG. 7 repeatedly appears depending on a color or luminance. As shown in FIG. 7, in the NTSC system, the phase of the color subcarrier SC is inverted every scanning in each field, and also inverted every frame. The phase inversion of the color subcarrier SC is described using various frequencies. Frequencies that are specified on the basis of standard NTSC system are represented by the following expressions (1) to (4).Standard field frequency fv=60 Hz/1.001=59.94 Hz  (1)Standard frame frequency fv/2=30 Hz/1.001=29.97 Hz  (2)Standard line frequency fH=262.5 fv=15.734 kHz  (3)Standard color subcarrier frequency fsc=227.5 fH=3.579545 MHz  (4)
The above expression (4) represents that the color subcarrier SC in one line has the frequency of 227 cycles+0.5 cycles (half-wavelength). That is, the color subcarrier SC of a target line is 0.5-cycle shifted from the color subcarrier SC of a subsequent line at the same position. Hence, the carriers of adjacent lines are 180° out of phase with each other. Such inter-line phase inversion is called “line interleaving”.
Further, the standard color subcarrier frequency fsc and the field frequency fv have a relation of the following expression (5).fsc=2×227.5×262.5 fv=119437.5 fv  (5)
The above expression (5) shows that a color subcarrier SC in one frame has a frequency of 119437 cycles+0.5 cycles (half-wavelength). That is, the color subcarrier SC shifts its phase by 0.5 cycles every frame, so the color subcarriers SC of adjacent frames are completely in opposite phase. Such inter-frame phase inversion is called “frame interleaving”.
A field sectional view of FIG. 9A shows a phase shift of the color subcarrier SC at the far right of each field in the field perspective view of FIG. 9A. In FIG. 9A, encircled numbers indicate a phase shift from a phase of the color subcarrier SC in the field “M+0” on line “N+1” at the far left; this phase is used as a reference. Here, “0.00” denotes “in phase”, i.e., no phase shift, and “0.50” denotes “in opposite phase”, i.e., a phase shift of 0.5 cycles. As mentioned above, the color subcarrier SC inverts its phase between lines in a field and between frames.
FIG. 9B shows a screen display image as viewed from the observer's eye in the sectional view of FIG. 9A. If the color subcarriers SC of adjacent lines or frames are in phase without the line- or frame-interleaving, crosstalk components resulting from the Y/C separation look like a stripe pattern or fixed pattern on the screen, so an observer has a difficulty in viewing an image. However, as shown in FIG. 9B, owing to the phase shift of FIG. 9A, level differences (peaks and troughs) of the color subcarrier SC appear leveled. Hence, as shown in FIG. 9B, a screen display image in a given color is obtained.
Referring next to FIGS. 10A to 10C, the display image is described in more detail. FIG. 10A shows display images of the respective fields (M+0-3) of FIG. 9A. FIG. 10B shows display images of two frames each obtained by combining two of the fields of FIG. 10A. FIG. 10C shows a display image obtained by superimposing the two frames of FIG. 10B. For ease of explanation, the peaks and troughs of the color subcarrier SC are represented in black and in white, respectively.
As shown in FIG. 10A, the color subcarriers SC of adjacent lines in each field are in opposite phase due to line interleaving, so the white and black portions appear in different patterns between adjacent lines, and each white portion and each black portion of adjacent lines overlap together; neither the white portions nor the black portions overlap each other. That is, as shown in FIG. 10B, a checkered pattern is obtained.
As shown in FIG. 10B, in one frame obtained by superimposing two fields, a line of the next field is fitted in between the lines of the target field, and the white portions and the black portions alternately appear every two lines to form a checkered pattern. Further, owing to frame interleaving, each white portion and each black portion of adjacent frames overlap together; neither the white portions nor the black portions overlap each other.
By superimposing the two frames, as shown in FIG. 10C, the white portions and the black portions completely overlap to obtain a display screen in a given color with an averaged brightness. That is, if an observer checks on the television screen the color subcarrier SC that inverts its phase between frames, these color subcarrier SC components would be perceived as flicker components of 15 Hz. Owing to the human eyes' space charge effect, the light and dark portions appear to cancel out in the eyes of the observer and thus, the luminance difference is hardly perceived.
As mentioned above, the NTSC system adopts the color subcarrier frequency fsc such as is line- or frame-interleaved to minimize an influence of various interferences.
Meanwhile, in the USA, an ATSC (Advanced Television Systems Committee)-compliant digital television broadcasting service has started from November, 1998 as the next-generation television broadcasting system in parallel with the NTSC-compliant broadcasting service. This ATSC system features “high definition/wide screen”, “high-quality sound”, “low noise”, “compatibility with various media”, and other such characteristics of a digital broadcasting system. Table 1 shows a result of comparing the ATSC system and the NTSC system as below.
TABLE 1Field frequency of transmissionDefinition ofside materialFieldresolutionDetails of definition(i: interlacing p: progressive)frequencyBroadcasting(imageverticalhorizontalAspect59.94p/59.94i/29.97p/23.976p/forsystemquality)(line)(pixel)ratio60p60i30p24poperationATSCHDTV size1080 1920 16:9—◯◯◯Both of(digital)(high7201280 16:9◯—◯◯59.94 Hzdefinitionand 60 Hzimage quality)SDTV size48070416:9◯◯◯◯(standard 4:3◯◯◯◯image quality)640 4:3◯◯◯◯NTSCSDTV size480640 4:3—◯——Only(analog)(standardequivalentequivalent59.94 Hzimage quality)(analog)◯: existing system
As shown in Table 1, in the ATSC system, 18 display systems from an SDTV (Standard Definition TV: standard quality) to the new system, HDTV (High Definition TV: high quality) are set for the moving pictures in varying combinations of a resolution (1920×1080, 1280×720, 704×480, and 640×480), an aspect ratio (16:9, and 4:3), and a field frequency fv (24 Hz, 30 Hz, and 60Hz). In the NTSC system, only one image quality, standard image quality, is defined.
The HDTV size and the SDTV size of the ATSC system are used for different applications based on the policy of a broadcasting station, a recoding size (resolution) of picture contents, and a theme of a TV program. For example, the HDTV size is used for contents requiring a high image quality such as movies, while the SDTV size is used for conventional NTSC-based broadcasting or used for concurrently delivering plural contents on account of requiring only a small data amount. Further, the size of the conventional NTSC system and the HDTV size of the ATSC system are different image sizes and thus incompatible.
Here, the field frequency fv means the unit representing how many fields (picture materials) are transmitted (displayed) per second. In the ATSC system, as shown in Table 1, there are two types as the field frequency fv, that is, NTSC-compatible 59.94 Hz series (23.976 Hz, 19.97 Hz, and 59.94 Hz) and 60 Hz series (24 Hz, 30 Hz, and 60.00 Hz). Nowadays, the US television broadcasting services are based on the two types of field frequencies.
Now, a description is given of why the ATSC system defines two field frequencies fv of 59.94 Hz series and 60 Hz series. The 59.94 Hz series is defined in conformity with the conventional standard NTSC system, and this frequency is intended for compatibility with the NTSC system. With this series, existing NTSC-compliant contents/devices (accumulated asset) can be used as they are, and it is easy to exchange contents between the NTSC system and the ATSC system. During a period of transition of the broadcasting system from the NTSC system to the ATSC system, devices of the two systems would be used, so this is extremely efficient. The reason the field frequency fv is not set to just 60.00 Hz but to a little smaller value, 59.94 Hz is that at the start of NTSC color broadcasting, the frequency fv of the color subcarrier SC is delayed from the frequency of a sound signal at a ratio of 1000/1001 in order to prevent the interference between the sound signal and the color subcarrier SC.
The 60 Hz series is 1.001 times (1,000 ppm) higher than the field frequency fv of the standard NTSC system, and is suitable for global distribution of broadcasting materials albeit sacrificing the compatibility with the standard NTSC system. The PAL system as an analog system adopted in Europe uses the field frequency fv of 50 Hz. As compared with a complicated field frequency fv ratio between NTSC system and PAL system (=59.94 Hz:50 Hz=1200:1001), the field frequency fv ratio between the ATSC system using the frequency of 60 Hz and the PAL system is as simple as 60 Hz:50 Hz=6:5. This facilitates the Europe-North America rate conversion. Further, it is said that the ATSC system using the frequency of 60 Hz is based on the fact the MUSE-compliant HDTV that has been developed in Japan prior to the ATSC system uses the frequency fv of 60 Hz, and its contents and devices can be easily applied. The ARIB system conforming to Japanese digital broadcasting standards is intended for the compatibility with the standard NTSC system, and standardizes the frequencies at 59.94 Hz series.
In this way, the ATSC broadcasting contents is distributed by selecting the HDTV size or SDTV size in accordance with its application and also setting the field frequency fv to 59.94 Hz or 60 Hz. For example, in the case where the content is distributed from the broadcasting station with the HDTV size, i.e., high resolution, a video signal decoded on the receiver side is accordingly based on the HDTV size. As mentioned above, the picture of the HDTV size is incompatible with the SDTV size of the conventional NTSC system. That is, unless the size is converted, it is impossible to input this video signal into a conventional NTSC television to view the picture or input a recording device (VCR/DVD recorder/HD recorder) to record the picture. Several years have elapsed from the start of the ATSC broadcasting service, and prices of an HDTV-ready TV monitor or HDTV recording device are coming down. Nevertheless, such devices are expensive, so the replacement of the device places great economical burden on a user. Therefore, in present developmental stages, it is desirable to enable the distribution of the ATSC broadcasting service to the user with little economical burden in consideration of the user's economical burden.
To that end, a video signal processing apparatus (set-top box) for converting the ATSC video signal received from the broadcasting station into the NTSC video signal has been widely used. As such a conventional video signal processing apparatus, there is known an apparatus disclosed in Japanese Unexamined Patent Publication No. 2004-208100.
FIG. 11 shows the configuration of a conventional video signal processing apparatus 900. The conventional video signal processing apparatus 900 converts an ATSC video signal input through an antenna 961 or a cable 962 into an NTSC video signal, and supplies the converted signal to an NTSC-compliant television receiver 970 or an NTSC-compliant recorder 980.
The conventional video signal processing apparatus 900 includes, as shown in FIG. 11, an ATSC decoder 910 for decoding an input transport stream (TS) into the ATSC video signal, a picture size converter 920 for converting an HDTV size into an SDTV size, an fv converter 930 for converting the 60 Hz-series field frequency fv to the 59.94 Hz-series field frequency fv, an NTSC encoder 940 for encoding the video signal of SDTV size into the NTSC video signal, and an fsc oscillator 950.
For example, if the input ATSC video signal has the field frequency of 60 Hz series, the picture size converter 920 converts the HDTV-size picture into an SDTV-size one, and the fv converter 930 converts the field frequency into 59.94 Hz. After that, the NTSC encoder 940 converts the video signal into the NTSC video signal based on the color subcarrier frequency fsc from the fsc oscillator 950.
If the input ATSC video signal has the field frequency of 59.94 Hz series, the picture size converter 920 converts the HDTV-size picture into an SDTV-size one, and the NTSC encoder 940 converts the ATSC video signal into the NTSC video signal based on the color subcarrier frequency fsc from the fsc oscillator 950 not through the fv converter 930.
However, the above conventional video signal processing apparatus 900 involves various drawbacks resulting from the provision of the fv converter 930. For example, the fv converter 930 stores the received image (field) into the fv converting buffer memory 931 and then converts the image into a desired field frequency fv to output the converted image, so a large memory capacity is required, which is uneconomical.
Further, the conversion ratio for the field frequency fv is as complicated as 60 Hz:59.94 Hz=1001:1000. This conversion based on the field frequency fv ratio means that one picture is removed from 1001 moving pictures to obtain 1000 moving pictures (referred to as “skipping”). The “skipping” takes place every 16.68 second, which value is calculated by converting 1001 to seconds (1001/60=16.68). As a result of “skipping”, the continuity of moving images is lost every 16.68 seconds, resulting in unnatural images with a deteriorated quality. The resulting image quality is not an intended quality, so there is a fear about the collision with the portrait rights.